1. Field of the Invention
The invention relates to a liquid crystal display device including a light-reflector and the light-reflector, and more particularly to a liquid crystal using an externally incident light as a light source by reflecting the externally incident light to a viewer by means of a light-reflector, and the light-reflector.
2. Description of the Related Art
A liquid crystal display device is grouped into a light-reflection type one, a light-transmission type one and a half-light-transmission type one in accordance with a light source thereof.
A light-transmission type liquid crystal display device includes a light source for transmitting backlight by which an image is displayed.
A light-reflection type liquid crystal display device includes a light-reflector therein, which reflects en externally incident light to a viewer. That is, the light-reflector acts as a light source. Accordingly, unlike a light-transmission type liquid crystal display device, a light-reflection type liquid crystal display device does not need to have a backlight source.
A half-light-transmission type liquid crystal display device is comprised of a combination of the above-mentioned light-transmission and light-reflection type liquid crystal display devices.
A light-reflection type liquid crystal display device consumes smaller power, and can be fabricated thinner and lighter than a light-transmission type liquid crystal display device. This is because it is possible in a light-reflection type liquid crystal display device to use an externally incident light as a light source by reflecting it at a light-reflector, and hence, it is not necessary for a light-transmission type liquid crystal display device to include a backlight source. A light-reflection type liquid crystal display device is widely used as a display device mainly in a terminal device such as a cellular phone.
A presently available light-reflection type liquid crystal display device is comprised of a liquid crystal of a twisted nematic (TN) type, a single polarizer type, a super twisted nematic (STN) type, a guest host (GH) type, a polymer diffusion liquid crystal (PDLC) type, or a cholesteric type, a switching device for driving a liquid crystal, and a light-reflector formed inside or outside a liquid crystal cell. A light-reflection type liquid crystal display device typically includes a thin film transistor (TFT) or a metal/insulator/metal (MIM) diode as a switching device in combination with a light-reflector, and is driven in accordance with an active matrix driving process which can accomplish high fineness and high image quality.
For instance, Japanese Patents Nos. 2825713 (B2) and 3012596 (B2) suggest a light-reflection type liquid crystal display device.
In the suggested light-reflection type liquid crystal display device, a light-reflector is formed at a surface thereof with a rugged pattern by the steps of forming an organic insulating film on the light-reflector, patterning the organic insulating film by photolithography and etching to thereby form isolated raised portions, forming an interlayer insulating film on the raised portions to thereby form a smooth rugged pattern having mountain portions comprised of the raised portions and valley portions comprised of portions other than the mountain portions.
FIG. 1 is a plan view illustrating an example of a rugged pattern formed on a conventional light-reflector.
As illustrated in FIG. 1, the illustrated rugged pattern has a plurality of raised portions 2 each having a circular cross-section, formed at a surface of the light-reflector 1. The raised portions 2 are formed isolated from one another.
The conventional light-reflector 1 has a purpose of scattering an incident light to some degree, and then, reflecting the scattered incident light. Hence, the scattered incident light is reflected almost uniformly such that the reflected light forms a cone.
FIG. 2 illustrates a relation between an incident light Li and a light Lr reflected at the light-reflector illustrated in FIG. 1.
As illustrated in FIG. 2, an incident light Li (for instance, a light emitted from a fluorescent lamp or a sunlight) entering a light-reflection type liquid crystal display device in a direction in which a viewer watches a display screen of the light-reflection type liquid crystal display device is reflected at the light-reflector 1, and is diffused almost uniformly in every direction as a reflected light Lr.
The light-reflector 1 having a rugged pattern comprised of the raised portions 2 each having a circular cross-section generally receives an intensive direct light in a particular direction such as a light emitted from a fluorescent lamp in a room. The light-reflector 1 cannot efficiently reflect a light entering in a particular direction, towards a viewer in an atmosphere in which the light-reflector 1 receives a weak indirect light having been reflected from a wall, and hence, a light-reflection type liquid crystal display device including the light-reflector 1 cannot make efficient use of an incident light entering the light-reflector 1. Accordingly, just a weak light is reflected to a viewer, and thus, the viewer feels that a display screen of a light-reflection type liquid crystal display device including the light-reflector 1 is dark.
In dependence on a shape of a rugged pattern formed at a surface of the light-reflector 1, lengths of paths along which lights run are different from one another in accordance with a location at which the light is reflected in the rugged pattern As a result, a tone would be remarkably varied due to interference caused by a difference among the above-mentioned lengths of paths, in dependence on an angle formed among a viewer, the light-reflector, and an incident light. This deteriorates display performance of a color liquid crystal display device.
In order to solve such a problem as mentioned above, the assignee of the present invention has suggested a light-reflector and a light-reflection type liquid crystal display device including the same in Japanese Unexamined Patent Publication 2002-258272 (A) published on Sep. 11, 2002. The suggested light-reflector can efficiently reflect a light towards a viewer.
The suggested light-reflector and light-reflection type liquid crystal display device including the same are explained hereinbelow. However, it should be noted that the explanation made hereinbelow does not mean that the applicant admits Japanese Unexamined Patent Publication 2002-258272 (A) as statutory prior art to the present invention. The Publication is explained only for the purpose of better understanding of the present invention.
FIG. 3 is a partial cross-sectional view of the suggested light-reflection type liquid crystal display device 10.
The suggested light-reflection type liquid crystal display device 10 is comprised of a lower substrate 11, an opposing substrate 12 arranged in opposing relation with the lower substrate 11, and a liquid crystal layer 13 sandwiched between the lower substrate 11 and the opposing substrate 12.
The light-reflection type liquid crystal display device 10 is of an active matrix type, and hence, includes thin film transistors (TFTs) in each of pixels as a switching device.
The lower substrate 11 is comprised of an electrically insulating substrate 14, an electrically insulating protection film 15, a thin film transistor 16, a first electrically insulating layer 17, raised portions 18, a second electrically insulating layer 19, and a reflection electrode 20.
The electrically insulating protection film 15 is formed on the electrically insulating substrate 14, and the thin film transistor 16 is formed on the electrically insulating protection film 15. The thin film transistor 16 is comprised of a gate electrode 16a formed on the electrically insulating substrate 14, a semiconductor layer 16c formed on the electrically insulating protection film 15, covering the gate electrode 16a therewith, a drain electrode 16b formed on the electrically insulating protection film 15, making electrical contact with the semiconductor layer 16c, and a source electrode 16d formed on the electrically insulating protection film 15, making electrical contact with the semiconductor layer 16c. 
The first electrically insulating layer 17 is formed covering the semiconductor layer 16c, the drain electrode 16b and the source electrode 16d therewith. The raised portions 18 are formed on the first electrically insulating layer 17 and the source electrode 16d. The second electrically insulating layer 19 covers the raised portions 18, the first electrically insulating layer 17 and the source electrode 16d therewith, and is formed with a contact hole 21 therethrough which reaches the source electrode 16d. 
The reflection electrode 20 is formed covering the contact hole 21 and the second electrically insulating layer 19 therewith. The reflection electrode 20 is electrically connected to the source electrode 16d of the thin film transistor 16, and acts as a light-reflector and a pixel electrode.
In a terminal region defined as a marginal region of the lower substrate 11, a gate terminal 22 is formed on the electrically insulating substrate 14, and a drain terminal 23 is formed on the electrically insulating protection film 15 partially covering the gate terminal 22 therewith.
The opposing substrate 12 is comprised of an electrically insulating substrate 26, a color filter 25 formed on the electrically insulating substrate 26, and a transparent electrode 24 formed on the color filter 25. The color filter 24 and the transparent electrode 24 face the liquid crystal layer 13.
The incident light Li entering the light-reflection type liquid crystal display device 10 through the electrically insulating substrate 26 reaches the lower substrate 11 through the liquid crystal layer 13, and then, is reflected at the reflection electrode 20 as the reflected light Lr. Then, the reflected light Lr is emitted out of the light-reflection type liquid crystal display device 10 through the liquid crystal layer 18 and the opposing substrate 12.
FIG. 4A illustrates the incident light Li directing to the light-reflector 1, and the reflected light Lr reflected at the light-reflector 1 and caught by a viewer.
Herein, an incident angle Ti is defined as an angle formed between the incident light L and a normal line of the light-reflector 1, and a reflection angle Tr is defined as an angle formed between the reflected light Lr and the normal line of the light-reflector 1. Since the incident light Li is reflected at the reflection electrode 20 formed in a rugged pattern by the raised portions 18 and the second electrically insulating layer 19, the incident angle Ti and the reflection angle Tr are different from each other.
FIG. 4B illustrates how a light reaching a point A of the reflection electrode 20 is reflected. For simplification, only a surface of the reflection electrode 20 and the light-reflector 1 are illustrated in FIG. 4B.
The incident light Li reaching the point A of the reflection electrode 20 having a rugged pattern is reflected at a tangential plane at the point A, and hence, the incident light Li is reflected in a direction symmetrical about a normal line of the light-reflector 20 at the point A.
Assuming that an inclination angle point A is defined as an angle formed between a tangential plane of the reflection electrode 20 at the point A and the light-reflector 1, a profile of the reflected light Lr is dependent on a profile of the inclination angles θ of a rugged pattern of the reflection electrode 20. Accordingly, since a viewer P subjectively evaluates a brightness of the light-reflector 1, it is important to design a profile of the inclination angles θ such that a viewer P feels that a display screen is bright.
Analyzing how a light-reflection type liquid crystal display device is used by a user, it is understood that, in almost all cases, a viewer P catches the reflected light Lr in such manners as illustrated in FIGS. 5A and 5B. In FIG. 5A, the incident light Li emitted from a light source S located at an angle between 0 to −60 degrees from a normal line of the light-reflector 1 is reflected in a direction angled from the normal line of the light-reflector 1 between −10 and 20 degrees. In FIG. 5B, the incident light Li coming in a direction angled between −20 and 20 degrees about the point A of the light-reflector 1 is reflected in a direction angled between −20 and 20 degrees about the point A of the light-reflector 1.
By designing a rugged pattern of the light-reflector 1 to have raised and recessed portions extending horizontally as viewed from a viewer, it would be possible to design the light-reflector 1 to have such a directivity that the incident lights Li emitted from the light source S are efficiently reflected to the viewer P as the reflected lights Lr.
FIG. 6 is a plan view of a rugged pattern formed at a surface of the light-reflector 1.
In FIG. 6, a hatched area indicates an area in which the raised portions 18 are formed, and hollow triangles indicate areas in each of which a recessed portion is formed. Though the hollow triangles indicating a recessed portion are arranged regularly, the triangles are actually arranged irregularly to a degree.
In FIG. 6, the raised portions 18 define three sides of each of the triangles. As an alternative, a plurality of linear raised patterns may define a closed shape such as a rectangle or an ellipse.
FIG. 7 is a cross-sectional view of the rugged pattern illustrated in FIG. 6.
In FIG. 7, L indicates a distance between centers of the adjacent raised patterns 18, W indicates a width of the raised pattern 18, D indicates a height of the raised pattern 18, d indicates a minimum height of the second electrically insulating layer 19, and ΔD indicates a difference between a maximum height of the second electrically insulating layer 19 and a minimum height of the second electrically insulating layer 19. Since the reflection electrode 20 comprised of an aluminum film coated on the second electrically insulating layer 19 is extremely thin, a thickness of the reflection electrode 20 is ignorable, and hence, is not illustrated in FIG. 7.
FIG. 6 illustrates the raised pattern 18 associated with a pixel.
In the above-mentioned conventional light-reflection type liquid crystal display device 10, the raised pattern 18 is uniquely determined in association with a pixel, and when a plurality of pixels are arranged, the thus determined raised pattern 18 is repeatedly arranged.
For instance, when three pixels 30a, 30b and 30c are successively arranged, as illustrated in FIG. 8, the raised pattern 18 illustrated in FIG. 6 is repeatedly arranged in association with every pixel 30a, 30b and 30c. 
In these days, a liquid crystal display device is required to have high fineness. In order to accomplish high fineness in a liquid crystal display device, a pitch of repetition of the raised patterns 18, that is, a width of a pixel has to be as small as possible. If a pixel is designed to have a smaller width, a width includes the smaller number of triangles or recessed portions.
When a liquid crystal display device is used for displaying a Black and white image in a portable device such as a cellular phone, the liquid crystal display device is generally designed as a light-reflection type one.
However, if a liquid crystal display device is used for displaying color images in a portable device, a light-reflection type liquid crystal display device is accompanied with a problem of shortage in a brightness. Hence, in these days, a liquid crystal display device used for displaying color images is designed as a half-light-transmission type one including a first area in which an incident light is reflected and a second area in which a back light transmits therethrough, in a single pixel in order to compensate for shortage of brightness. Such a half-light-transmission type liquid crystal display device includes a backlight source for the second area, and hence, makes it possible to display bright images even in dark atmosphere.
FIG. 9 is a plan view of a half-light-transmission type liquid crystal display device.
Similarly to the light-reflection type liquid crystal display device illustrated in FIG. 8, the illustrated half-light-transmission type liquid crystal display device includes three pixels 300a, 300b and 300c. Each of the pixels 300a, 300b and 300c is designed to have a light-reflection area 301a, 301b and 301c in which a light-reflector is formed, and a light-transmission area 302a, 302b and 302c in which a light passes therethrough, respectively. Each of the light-reflection area 301a, 801b and 301c occupies about one-third in an area of the pixels 300a, 300b and 300c, and each of the light-transmission area 302a, 302b and 302c occupies two-thirds in an area of the pixels 300a, 300b and 300c. 
A rate of a light-reflection area and a light-transmission area in a pixel is dependent on a device to be used, but, an area occupied by a light-reflector in a pixel in a half-light-transmission type liquid crystal display device is smaller than an area occupied by a light-reflector in a pixel in a light-reflection type liquid crystal display device. Accordingly, the number of triangles or recessed portions included in a pixel in a half-light-transmission type liquid crystal display device is smaller than the same in a light-reflection type liquid crystal display device.
An area of a triangle or a recessed portion is dependent on a capacity of an apparatus for fabricating a liquid crystal display device, such as an accuracy with which a photoresist is exposed to a light, and hence, an area of a triangle or a recessed portion cannot be formed too small.
As mentioned above, reduction in the number of triangles or recessed portions to be included in a pixel, due to requirement to high fineness and tendency of fabrication of a liquid crystal display device as a half-light-transmission type one, causes a problem of interference of the reflected lights Lr with each other (see FIG. 2). This is because if the number of triangles or recessed portions included in a pixel is reduced, it would be difficult to cancel interference of the reflected lights with each other in a pixel.
A light-reflector is generally designed to have a pattern of repeating a pixel, and hence, if it is not possible to cancel interference of the reflected lights with each other in a pixel, it would not be possible to cancel interference of the reflected lights with each other also in a plurality of pixels. Interference of the reflected lights Lr with each other would degrade display performance of a liquid crystal display device.